A pile driver is used to drive long slender columns, referred to as "piles", into the ground so that the pile can be used to support a vertical load. Such driving is accomplished by repeatedly striking the top of the pile with a downwardly-moving device, referred to as a "ram". Each time the ram strikes the pile, a substantial portion of the energy of the ram is imparted to the pile whereby the pile moves downwardly into the ground. Knowledge of the energy possessed by the ram at the time it strikes the pile provides substantial useful information regarding the dynamics of the pile in the ground and the ability of the pile to support a load. For example, as set forth in U.S. Pat. Nos. 3,498,388, issued to Jovis on Mar. 3, 1970, and 2,580,299, issued to Hunicke on Dec. 25, 1951, the knowledge of such energy is useful in determining the safe bearing capacity of the pile in accordance with the partly empirical Engineering News formula.
The energy delivered by a hammer for each blow, i.e., for each time the ram of the hammer strikes the pile, is a function of various parameters such as piston weight, blow rate, i.e., the number of blows per unit time, and stroke, i.e., the distance through which the piston falls for each blow. For some types of hammers, the stroke and the piston weight is constant for a particular hammer model of a manufacturer. For such hammer types, the energy delivered on each blow can be determined from the blow rate in accordance with a table prepared by the manufacturer. See Jovis, col. 3, lines 6-33. For open-ended diesel hammers, however, the stroke is not constant. Therefore, in order to determine the energy delivered for each blow by an open-ended diesel hammer, the stroke of the blow must be determined.
In the prior art, devices used for measuring the variable parameters related to the dynamics of pile driving hammers, such as blow rate and stroke, have included mechanisms attached to the hammer itself or to the site being driven by the hammer. In view of the substantial force delivered by the hammer, such devices are subject to rapid wear and tear and to damage. Furthermore, such devices are costly to install and may interfere with the pile driving operation by requiring attachment to and detachment from each pile prior to and after driving and by requiring cables running to and from the hammer and/or pile. In addition, such devices do not provide information by which the stroke is determined for open-end diesel hammers and the blow rate is determinable for other types of hammers.